1. Field of the Invention
The present invention relates to a magnetoresistive device and a magnetic random access memory.
2. Related Art
Various types of solid magnetic memories have been suggested. In recent years, magnetic random access memories (MRAM) that use magnetoresistive devices showing the giant magnetoresistive (GMR) effect have been suggested, and attention has been drawn particularly to magnetic random access memories that use ferromagnetic tunnel junctions showing the tunneling magnetoresistive (TMR) effect.
A MTJ (Magnetic Tunnel Junction) device including a ferromagnetic tunnel junction includes a stack structure normally formed with a first ferromagnetic layer, an insulating layer, and a second ferromagnetic layer. At the time of reading, current flows, tunneling through the insulating layer. In this case, the junction resistance value varies with the cosine of the relative angle between the magnetization direction of the first ferromagnetic layer and the magnetization direction of the second ferromagnetic layer. Accordingly, the resistance value of the ferromagnetic tunnel junction is smallest when the magnetization directions of the first ferromagnetic layer and the second ferromagnetic layer are parallel to each other, and is largest when the magnetization directions of the first ferromagnetic layer and the second ferromagnetic layer are antiparallel to each other. This is called the TMR effect. The change rate of the resistance value due to the TMR effect sometimes exceeds 300% at room temperature.
In a memory cell that includes the ferromagnetic tunnel junction as a ferromagnetic memory device, at least one of the ferromagnetic layers is regarded as a magnetic reference layer (also referred to as a fixed magnetization layer, a reference layer, or a pinned layer), and the magnetization direction of the ferromagnetic layer is fixed. The other one of the ferromagnetic layers is regarded as a recording layer (also referred to as a magnetic recording layer, a free layer, or a variable layer). In this memory cell, the magnetization directions of the magnetic reference layer and the magnetic recording layer are in a parallel state or in an antiparallel state, and the binary information of “0” and “1” is associated with the parallel state and the antiparallel state. In this manner, information is written. To write recording information, the magnetization of the magnetic recording layer is reversed by the magnetic field generated by the current flowing into a write wiring provided for this memory cell (a current magnetic-field reversal method). Alternatively, the magnetization of the magnetic recording layer is reversed by the spin torque injected from the magnetic reference layer by directly energizing the device (a spin-injection magnetization reversal method (see U.S. Pat. No. 6,256,223, for example)). Reading is performed by applying current to the ferromagnetic tunnel junction and detecting a resistance variation caused by the TMR effect. A large number of such memory cells are arranged, to form a magnetic memory. In an actual structure, a switching transistor is provided in each of the cells as in a DRAM, and peripheral circuits are incorporated into the structure, so that any desired cell can be selected.
To realize a large-capacity memory, it is necessary to miniaturize each device, and increase the cell occupancy in the chip. The spin-injection magnetization reversal method requires a much smaller amount of current for writing information than the conventional current magnetic-field reversal method. For this reason, the spin-injection magnetization reversal method is a write method suitable for realizing a large-capacity magnetic memory.
When a magnetic memory of the spin-injection magnetization reversal type is used in practice, the write current applied to the devices should preferably not change with temperature. This is because, if the write current has large temperature dependence, it is necessary to prepare a circuit that adjusts the write current every time the temperature fluctuates, and the cell area of the memory becomes larger.
In a conventional magnetic memory of the current magnetic-field reversal type that utilizes the shape magnetic anisotropy of a magnetic film having magnetization substantially parallel to the film plane (in-plane magnetization), the magnetic anisotropy changes in proportion mainly with the saturation magnetization. Since the temperature dependence of the saturation magnetization of the magnetic layer is expressed as a Brillouin function, the temperature dependence is small at temperatures close to room temperature. Accordingly, the temperature dependence of the magnetic anisotropy is small, and the temperature dependence of the current magnetic field required for a reversal is also small, which has not raised any problem.
In a magnetic memory of the spin-injection magnetization reversal type that utilizes the magnetization substantially perpendicular to the film plane (perpendicular magnetization), however, the energy required for a spin injection reversal changes in proportion to the magnetic anisotropy of each device. Therefore, it is difficult to guarantee a temperature range of below zero to 150° C. as required for an in-vehicle memory, for example. This still remains a problem.